1. Field of the Invention
The present invention relates to a carrier used for developing a latent image created in electrostatic photography and electrostatic recording, a developer using this carrier, an image-forming method using this developer, and an image-forming method used in an image-forming apparatus such as a digital printer or a digital copier which processes images as digital signals.
2. Description of the Related Art
In a digital-image-forming apparatus based on letters or image data, on/off binary information is provided as two-dimensional information at a predetermined site on a photoreceptor. When a halftone image is recorded by the above-mentioned system, an area modulation method, which uses a mesh or line screen structure, was conventionally adopted in many printers and copiers based on digital photography because of the relative easy algorithm and low cost involved.
Meanwhile, a method in which image information is visualized via a latent image, e.g., electrostatic photography, is now widely used. Electrostatic photography comprises the steps of forming a latent image on a photoreceptor by electrostatic charging and exposure, developing the latent image using a developer containing toner, transferring the toner image, and fixing the transferred toner image to form a visible image on an image-receiving medium. There are two types of developer, i.e., a two-component developer composed of toner and carrier and a one-component developer composed of a single magnetic toner. The two-component developer, whose role is allotted to the carrier and toner, has superior control and is therefore widely used.
In the developing process used in an image-forming apparatus for reproducing a multiple gradation via electrostatic photography, the cascade method has been superseded by a magnetic brush method in which a magnetic roller is used as a developer carrying member. Particularly, in the case of a color image-forming apparatus, because of the stabilized charge of the developer, a magnetic brush method using a two-component developer, which comprises a carrier and toner, is more suitably employed.
Two types of magnetic brush method using a two-component developer are known: the conductive magnetic brush development (CMB), which uses a conductive carrier, and the insulated magnetic brush development (IMB), which uses an insulated carrier. In CMB development, because of the carrier""s lower resistance level, the electrical charge is injected from a developing roller so that the carrier near the photoreceptor serves as a developing electrode to enhance a practical electric field for development. As a result, since toner is sufficiently transported, a superior solid image free of the edge effect can be reproduced. However, CMB development has the disadvantage that, since the relationship between latent image potential and image density of the photoreceptor changes abruptly as indicated by the steep slope of the curve, image defects such as white lines called brush marks, i.e., latent image destruction due electrical charge injection from the developing roller, and so-called carrier beads carry over, i.e., migration of the carrier to the photoreceptor, tend to occur. On the other hand, in IMB development, the relationship between latent image potential and image density of the photoreceptor is linear and has a gentle slope. The disadvantages of IMB development are that solid images are poorly filled and that the edge effect is significant.
If the degree of the above defects is insignificant, black-and-white images, which are formed by black toner alone, are not seriously influenced in sensory inspection. The above defects, however, present a fatal drawback to color images formed by the overlap of multiple toner colors. This is because, in a color image, these defects cause a slight change in color, causing xe2x80x9cnoisexe2x80x9d due to different colors in a gradated range, even though these defects are only a slight change in density in black-and-white images. Accordingly, these defects extremely adversely affect the impression of color images in particular.
Because of this background, conductive magnetic brush methods, which exhibit superior performance in terms of filling of solid image, edge effect, carrier beads carry over and brush marks, have been disclosed.
For example, Japanese Patent Application Publication (JP-B) No. 7-120,086 discloses a method based on the electrical resistance of a carrier, which comprises a core having a relatively low electrical resistance coated with a resin having a high electrical resistance which abruptly changes in an electric field having a specific strength, causing the electrical resistance of the carrier to increase in a weak electric field, whereas the electrical resistance of the carrier decreases in a strong electric field. Based on this disclosure, since a stronger electric field exists in the latent image area and a weak electric field exists in the nonimage area, use of this carrier enables superior solid black image printing without carrier beads carry over to the nonimage area. However, based on the examples and description of the function of the invention in the above Japanese Patent Application Publication (JP-B) No. 7-120,086, the resin coating layer is so thin that a core having a lower electrical resistance is believed to be partly exposed, which leads to lower electrical resistance of the carrier under a strong electric field. To substantiate this assumption, comparative examples, which are described later, verify that the electrical resistance of a carrier, produced by completely coating a core with a thick layer of resin, exhibits a higher electrical resistance even in a strong electric field and does not provide a superior solid image. The partial coated carrier, which has a partly exposed core having lower electrical resistance, tends to cause brush marks in the latent image because the electrical charge moves easily via exposed surfaces.
Japanese Patent Application Laid-Open (JP-A) No. 61-107,257 and Japanese Patent Application Laid-Open (JP-A) No. 61-130,959 disclose a ferrite which has a relatively low electrical resistance and has surface roughness caused by primary particles. According to the disclosure, because of carrier particle roughness, leakage between oppositely polarized charges is inhibited so that brush mark formation is inhibited. The disadvantage, however, is that the presence of the roughness on the carrier surface increases the contact area between the carrier and toner so that toner adheres more to the carrier surface, which diminishes the charge-imparting capability of the carrier over time.
Japanese Patent Application Laid-Open (JP-A) No. 6-161,157 specifies the ratio of the electrical resistance of the core to the electrical resistance of the resin-coated carrier itself so that the carrier provides superior resolution, proper solid image density, and fine line reproduction. No remarked effect is seen, however, in preventing image defects, particularly in color images.
As stated above, none of the available carriers and image-forming methods perform satisfactorily in view of recent stringent requirements for high-quality images, including color images, because existing carriers and image-forming methods do not solve the problems of image defects associated with the conductive magnetic brush, namely, the problems of carrier beads carry over and brush marks caused by the destruction of the latent image due to bias leakage.
Accordingly, one object of the present invention is to provide a carrier, which produces a superior solid image free of brush marks and carrier beads carry over, particularly in color images, and which is durable for a long time, and to provide a developer using this carrier and, further, to provide an image-forming method using this developer.
Another object of the present invention is to provide an image-forming method which have the advantage of stabilizing the amount of toner moving to a latent image, even if the photoreceptor is not uniformly sensitive, producing halftone images having excellently filled solid images free of edge effects and brush marks and preventing carrier beads carry over.
To solve the above problems, the present inventors have conducted studies and found that, in order to obtain a superior solid image by preventing image defects such as brush marks and carrier beads carry over, the electrical resistance of the carrier must be within a specific range, and that this condition can be attained by using a carrier core having an electrical resistance not exceeding a specific value and a resin coating layer having an electrical resistance falling within a specific range.
Further, to obtain a developer which provides a superior solid image free of the edge effect and of carrier beads carry over and brush marks by adjusting the electrical resistance of the resin-coated carrier, the present inventors found that it is necessary to use a developer having a saturated region in the developing curve defined by a contrast potential, which is determined by bias potential for development and the potential at the exposed part on a latent image substrate, and the amount of developer toner moving to the latent image on the latent image substrate, in order to stabilize the amount of developer toner and that, even if the electrical resistance of the resin-coated carrier itself is the same, the saturation characteristic varies with the electrical resistance of the core. As a result, they achieved an invention based on these findings.
That is, the carrier according to the present invention comprises a core and a resin coating layer containing an electroconductive powder formed on the core, wherein, when the magnetic brush is formed only of the core, the dynamic electrical resistance of the core forming the magnetic brush under an electric field of 104 V/cm is 1 xcexa9xc2x7cm or less and the electrical resistance of the resin coating layer is from 10 to 1xc3x97108 xcexa9xc2x7cm.
The developer according to the present invention comprises the above-described carrier and toner containing a binder resin and a colorant.
The image-forming method according to the present invention comprises the steps of forming a latent image on a latent image substrate, developing the latent image using a developer, transferring the developed toner image to an image-receiving medium, and thermally fixing the toner image on the image-receiving medium, wherein the developer is the above-described developer.
In this image-forming method, developing the latent image can be carried out by means of a developer held on a developer carrying member provided with a bias potential for development, and a developer having a saturated region in the developing curve defined by a contrast potential, which is determined by a bias potential for development and the potential at the exposed part on the latent image substrate, and the amount of developer toner moving to the latent image on the latent image substrate, can be used and the bias potential for development can be applied to the developer carrying member so that the amount of developer toner exhibits the saturation characteristic.
The carrier of the present invention comprises a core having a low level of electrical resistance, which is indicated by a dynamic electrical resistance of 1 xcexa9xc2x7cm or less as measured in a form of a magnetic brush formed only by the core under an electric field of 104 V/cm, and a resin coating layer having an intermediate level of electrical resistance which is from 10 to 1xc3x97108 xcexa9xc2x7cm.
The use of the above structure makes it possible to simultaneously achieve two objectives, i.e., to obtain a superior solid image and to prevent defects such as brush marks and carrier beads carry over. The mechanism is presumably as follows: Generally, if a conductor is placed in an electric field, the electrical charge is reoriented, i.e., polarization occurs. The speed of polarization depends on the resistance of the conductor, i.e., the lower the resistance, the faster the polarization. This phenomenon occurs within the core which is placed between the developing roller and the photoreceptor. If core resistance is so low that the core becomes polarized within about 10xe2x88x923 second, which is the time interval for development, a superior solid image can be obtained, because the formation of the developing electrode by the polarization of the core itself in addition to charge injection from the developing roller will act advantageously. However, a superior solid image cannot be obtained if the total electrical resistance of the carrier increases due to the high electrical resistance of the resin coating layer, even if the electrical resistance of the core is low. Meanwhile, since the electrical charge injected from the developing roller flows mainly through the surface of the carrier, brush marks and carrier beads carry over tend to occur, if the electrical resistance of the resin coating layer is too low. In the present invention, since the electrical resistance of the resin coating layer is within a specified range, it is possible to obtain a superior solid image without brush marks or carrier beads carry over occurring.
There is no theoretically established explanation for the fact that a satisfactorily saturated region can be obtained when the electrical resistance of the core is low, even if the electrical resistance of the entire carrier is relatively high. Presumably, the mechanism is as follows: If the electrical resistance of the carrier is high, saturation cannot be easily attained, because the slope at the start of the developing curve is gentle due to a weak electric field for development and this condition causes an electric field within the layer of the developer and sends the toner to the latent image for development both from the surface layer of the developer and from the inside of the developer layer. On the other hand, if the electrical resistance of the carrier is low, the amount of developer toner becomes saturated, because the slope at the start of the development curve is so steep due to a strong electric field for development that the inside of the developer layer is almost electroconductive and therefore has no electric field and because only the toner, which is present on the developer layer, is used for development. In other words, if the electrical resistance of the core is low, the developing electrode is formed near the latent image substrate, which produces the saturated region.
Further, since low electrical resistance of the core effectively polarizes the core itself, the saturated region is understood to be obtainable, even if the electrical resistance of the carrier is high, as opposed to when the electrical resistance of the core is high.